1. Field of the Invention
The present invention relates to the selective hydrogenation of diolefins and acetylenic compounds and isomerization of olefins to more desirable isomers in an olefin rich stream containing sulfur impurities. More particularly the invention relates to a process utilizing hydrogenation catalysts in a structure to serve as both the catalyst and as a distillation structure for the simultaneous reaction and separation of the reactants and reaction products.
The present process includes the removal of mercaptans, hydrogen sulfide (H.sub.2 S) and polyolefins from petroleum distillate streams. More particularly the invention relates to a process wherein the petroleum distillate contains diolefins which are selectively reacted with the mercaptans and/or hydrogen sulfide (H.sub.2 S) to form sulfides and the remaining diolefins and acetylenes are hydrogenated to mono-olefins. More particularly the invention relates to a process wherein the reaction of the mercaptans and/or hydrogen sulfide (H.sub.2 S) with the diolefins is carried out simultaneously with a fractional distillation to remove the sulfides, and thus the sulfur, from the distillate. Most particularly the invention relates to a process wherein most of the hydrogen sulfide is removed prior to the reaction of the diolefins and mercaptans.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the compositions. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The petroleum distillates often contain unwanted contaminants such as sulfur and nitrogen compounds. These contaminants often are catalyst poisons or produce undesirable products upon further processing. In particular the sulfur compounds can be troublesome. The sulfur compounds are known catalyst poisons for naphtha reforming catalysts and hydrogenation catalysts. The sulfur compounds present in a stream are dependent upon the boiling range of the distillate. In a light naphtha (110.degree.-420.degree. F. boiling range) the predominant sulfur compounds are mercaptans. Streams having C.sub.3 hydrocarbons also may contain H.sub.2 S. The most common method for removal of the H.sub.2 S is amine extraction and the most common method for removal of mercaptans is caustic washing of the organic streams.
Another method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:
(1) RSH+H.sub.2 .fwdarw.RH+H.sub.2 S PA1 (2) RCl+H.sub.2 .fwdarw.RH+HCl PA1 (3) 2RN+4H.sub.2 .fwdarw.RH+NH.sub.3 PA1 (4) ROOH+2H.sub.2 .fwdarw.RH+H.sub.2 O PA1 (1) feeding hydrogen and a portion of the residual to distillation column reactor containing a first bed comprising a first hydrogenation catalyst of the type characterized by nickel, cobalt or iron, preferably selected from nickel, cobalt, iron or mixtures thereof, and prepared in the form of a distillation structure and a second bed position in said column above said first bed, said second bed comprising a second hydrogenation catalyst of the type characterized by platinum, palladium or rhodium, preferably selected from platinum, palladium, rhodium or mixtures thereof, and prepared as a distillation structure wherein any sulfur compounds in the residual react in said first bed with a portion of the diolefins to form sulfides in a first reaction mixture, PA1 (2) fractionally distilling the first reaction mixture to remove the sulfides with a heavier fraction and passing a lighter fraction into the second bed PA1 (3) hydrogenating the diolefins and acetylenes in said second bed to form a second reaction mixture PA1 (4) fractionally distilling the second reaction mixture and PA1 (5) removing a fraction overhead, which is substantially free of sulfur compounds, acetylenes and diolefins.
Typical operating conditions for the HDS reactions are:
______________________________________ Temperature, .degree.F. 600-780 Pressure, psig 600-3000 H.sub.2 recycle rate, SCF/bbl 1500-3000 Fresh H.sub.2 makeup, SCF/bbl 700-1000 ______________________________________
As may be seen the emphasis has been upon hydrogenating the sulfur and other contaminating compounds. The sulfur is then removed in the form of gaseous H.sub.2 S, which in itself is a pollutant and requires further treatment.
In addition to sulfur and nitrogen compounds mixed refinery streams contain a broad spectrum of olefinic compounds. This is especially true of products from either catalytic cracking or thermal cracking processes. These unsaturated compounds comprise ethylene, acetylene, propylene, propadiene, methyl acetylene, butenes, butadiene, amylenes, hexenes etc. Many of these compounds are valuable, especially as feed stocks for chemical products. Ethylene, especially is recovered. Additionally, propylene and the butenes are valuable. However, the olefins having more than one double bond and the acetylenic compounds (having a triple bond) have lesser uses and are detrimental to many of the chemical processes in which the single double bond compounds are used, for example polymerization. Over the range of hydrocarbons under consideration, the removal of highly unsaturated compounds is of value as a feed pretreatment, since these compounds have frequently been found to be detrimental in most processing, storage and use of the streams.
In the production of tertiary amyl methyl ether (TAME) for use as a gasoline additive generally a light cracked naphtha (LCN) is used as the source of the olefins for the etherification reaction. The acetylenes and diolefins are detrimental in the etherification process as well as in other processes such as alkylation and should be removed early in the stream processing. The LCN usually contains sulfur as a contaminant in the form of mercaptans in concentrations of up to hundreds wppm. These mercaptans are inhibitors for the hydrogenation catalyst used to hydrogenate dienes and acetylenes and obtain beneficial isomerization in the feed to an etherification unit or to an alkylation unit.
Although the most desirable hydrogenation catalysts are inhibited by sulfur compounds even in very small amounts, e.g. 10-100 ppm, there are other similar catalysts that will cause the sulfur compounds and the diolefins to form adducts, which can be separated from the lighter components.
It is an advantage of the present invention that the sulfur compounds can be separated from the lighter hydrocarbon components which can then be hydrotreated with the sulfur sensitive catalyst to hydrogenate highly unsaturated hydrocarbons and obtain beneficial isomerization of the mono-olefins. It is a particular advantage that this may be achieved in a single reactive distillation column by using beds of function specific catalyst. It is a particular feature of the present invention that a dual bed system may be used.